1. Field of the Invention
The present invention relates to power take off units for motor vehicles and more particularly to compressed air actuated clutch type power take off units.
2. Description of the Problem
Trucks and other commercial vehicles often come equipped with power take off (PTO) units operated at the election of the driver or operator. A (PTO) unit is coupled to the vehicle drive train using a clutch which is pressure actuated. On vehicles equipped with standard (manual) transmissions, compressed air from the vehicle's brake system is often used to close and clamp the PTO clutch. The air pressure available limits the clamping force generated. As a result, the maximum torque available through the clutch is limited by available air pressure. If the clamping force is too low for the PTO load, the clutch will slip resulting in damage to the clutch and a failure to carry the load on the PTO system. In the prior art pressure sensitive cut off valves have been incorporated into an air line between the brake system, compressed air system and the PTO unit to prevent operation of the PTO unit at low pressures.